A tube produced by various tube making methods is subjected to a finishing process, thereafter being applied with treatments such as heat treatment as needed, and becomes an end product after undergoing an inspection and testing process. The straightening of tube is one of the processing which is conducted in the finishing process, and has its object to correct bends in a tube along its axial direction and oval shapes of the cross section of the tube, which result from the straightening of the bends.
For straightening a tube, generally, a press type machine or an inclined roll type straightening machine (a rotary straightener: hereafter, simply referred to as a “straightener”) in which a plurality of concave type rolls are combined is used. There are a large number of configurations for a straightener resulting from the combinations of the number, and the arrangement and disposition of rolls. The description of the press machine is omitted since it is well known.
FIG. 1 is a diagram showing an example of the roll arrangement of a straightener. The straightener shown includes three pairs of straightening rolls: Ra1 and Rb1 (No. 1 stand), Ra2 and Rb2 (No. 2 stand), and Ra3 and Rb3 (No. 3 stand), each paired rolls being oppositely disposed with the directions of their rotational axes crossing with each other, and an auxiliary roll Rc (No. 4 stand (final stand)). The auxiliary roll Rc is a roll for enhancing the straightening effect by adjusting it vertically. These are also collectively called as straightening rolls. The straightener having the roll arrangement illustrated as an example in FIG. 1 is a 2-2-2-1 type straightener.
FIG. 2 is a diagram to explain the shape of a straightening roll to be used in a straightener, showing a roll longitudinal section (only an upper half above the roll axis) taken along any plane passing the roll axis. As shown in FIG. 2, the straightening roll has a so-called concave shape, and comprises roll shoulder portions, which stand at opposite ends, and a roll barrel portion having a concave surface which is curved from opposite roll shoulder portion toward the axial centerline of the roll starting from both the roll shoulder portions and to the middle of the roll (groove bottom P). The roll diameters D1 and D2 at major size roll shoulder portions are equal, and the curved surfaces forming the roll shoulder portions and the curved surfaces forming the barrel portion are symmetrical with respect to the length-wise middle of the roll (groove bottom P). That is, a conventional straightening roll has a symmetric shape.
In above described FIG. 1, it is possible to adjust the angle of the straightening roll R with respect to a base pass line (a roll angle which is required to make a workpiece move spirally) and a distance between opposite rolls (crush amount) of the paired straightening rolls R, respectively. Further, it is also possible to adjust, for example, the heights of central axes for the straightening rolls Ra2 and Rb2 of No. 2 stand in a vertical direction as an offset amount, with respect to the central axis for the straightening rolls Ra1 and Rb1 of No. 1 stand.
Generally, when straightening a tube by a straightener, the angle of each straightening roll R with respect to the tube 1 to be straightened (that is, a roll angle) is adjusted such that the surface of the tube 1 to be straightened lies along the surface of the straightening roll R. Further, the distance between respective straightening rolls R making up a pair at each stand is set to be slightly smaller than the outer diameter of the tube 1 to be straightened, thereby imposing pressure (crushing) on the tube, and the central axes of the straightening rolls Ra2 and Rb2 are made to be higher than (offset from) the central axes of the straightening rolls Ra1 and Rb1, and the straightening rolls Ra3 and Rb3, thereby giving a bending stress to the tube, to correct bends. That is, when straightening a tube by a straightener, it is necessary to appropriately adjust the roll angle, the crush amount, and the offset amount, which are setup conditions.
FIG. 3 is a diagram to explain the roll angle among the setup conditions for roll straightening. As shown in the figure, an angle θ formed between the axial centerline of a tube to be straightened and the central axis of the straightening roll R is the roll angle (degree). In the illustrated example, the straightening roll R is disposed below the tube 1 to be straightened, and the tube 1 moves to the direction shown by an outlined arrow by the rotation (rotation in the direction shown by the arrow) of the straightening roll R.
FIG. 4 is a diagram to explain the crush amount among the setup conditions for roll straightening. As shown in FIG. 4, the tube 1b to be straightened, which is applied with crush by roll straightening, is pressed to be deformed into an oval shape. In FIG. 4, a tube to be straightened (shown by a dashed line) before being applied with crush is denoted by a reference character 1a, and a tube to be straightened after being applied with crush is denoted by a reference character 1b. A crush amount c (mm) is shown by a difference between an original outer diameter d of the tube 1a to be straightened, before deformation, and a distance between paired straightening rolls Ra and Rb, and corresponds to the roll draft applied to the outer diameter of the tube 1 to be straightened. The tube 1 to be straightened is subjected to straightening of bends by being repeatedly pressed over its overall length while being rotated by the straightening rolls R.
FIG. 5 is a diagram to explain the offset amount among the setup conditions for roll straightening. As shown in the figure, the central axes of straightening rolls Ra2 and Rb2 are set (offset) to be higher than the central axes of straightening rolls Ra1, Rb1 in a first stand (on the near side (entry side) relative to the moving direction of the tube to be straightened). The offset amount δ (mm) is indicated by a displacement amount of the central axes of the straightening rolls Ra2 and Rb2 in a height direction (the direction of pressing). Further, the central axes of the straightening rolls Ra3 and Rb3 are at a lower level (which may not be the same level of the central axes of the straightening rolls Ra1 and Rb1) than that of the central axes of the straightening rolls Ra2 and Rb2 in a second stand, and are in a state of being inversely offset with respect to the straightening rolls Ra2 and Rb2. That is, straightening of bends is performed by alternately applying an upward and downward bending stress to the tube 1 to be straightened.
As described above, when performing straightening by a straightener, it becomes necessary to apply a certain level of load such as crushing and offsetting to the tube to be straightened. Therefore, studies on the method of setting an offset amount, a crush amount, and the like have been conducted up to now.
For example, Patent Literature 1 describes a method for setting an offset amount, a crush amount, and the like, in which an offset amount of the roll is determined based on a predetermined relationship between an index indicating a plastic deformation that is caused by offsetting in a tube section at an offset position, and the offset amount; and a crush amount of the roll is determined based on the predetermined relationship between an index indicating a plastic deformation that is caused by crushing in a tube section at a crush position, and the crush amount.
Meanwhile, if straightening processing is performed by setting an offset amount based on Patent Literature 1, there may be a case where a leading-end deformation (the front end portion of the tube to be processed) occurs. This is caused by that, when a tube is obliged to pass through the straightening rolls which are offset, the front end of the tube is liable to miss the engagement in between the upper and lower rolls, and collides with the rolls, thereby being subjected to an impact. The leading-end deformation occurs particularly when the offset amount is set to be larger (larger offsetting). Since occurrence of leading-end deformation causes the outer diameter to decrease, the affected part must be cut off thereby deteriorating the productivity.
Patent Literature 2 describes a method in which as a countermeasure for preventing the leading-end deformation, that is, a countermeasure for avoiding the collision of the front end of the tube to be processed against the roll, the distance of opposite rolls (the distance between the upper and lower rolls) of the roll pair (opposite rolls) which are oppositely disposed in a vertical direction is widely opened in advance, and if the front end of a tube enter between the opposite rolls, the upper roll which has been retracted upwardly is lowered to apply crushing (pressing rolling is started).
However, in such a method, since the front end of the tube will pass through the straightening roll before crushing is applied, straightening of the tube end portion will not be achieved. Further, it requires complex and highly accurate control. On the other hand, although there is a method in which the straightening condition is mitigated such as by setting a smaller offsetting, as well as a method in which the impact between the roll and the tube to be straightened is suppressed by enlarging the roll angle, the straightening force becomes weak in these methods, and thereby the straightening effect declines so that bends in the tube may not be sufficiently removed causing some of them to remain. If such bends remain, a separate re-straightening processing such as one to remove bends by using a press machine arranged in an off-line, or one to remove bends by passing the tube through the straightener again becomes necessary, thus deteriorating the productivity.